Target touches to the correct display.

ui/events/event.h

Index: ui/events/event.h
diff --git a/ui/events/event.h b/ui/events/event.h
index 596f31e78b10ec6d45d8b2edca68f047b986fdbb..55687035af681322f0a0cd10259d19661829646c 100644
--- a/ui/events/event.h
+++ b/ui/events/event.h
@@ -437,7 +437,8 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
         radius_x_(model.radius_x_),
         radius_y_(model.radius_y_),
         rotation_angle_(model.rotation_angle_),
-        force_(model.force_) {
+        force_(model.force_),
+        source_device_id_(model.source_device_id_) {
   }
 
   TouchEvent(EventType type,
@@ -462,6 +463,7 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
   float radius_y() const { return radius_y_; }
   float rotation_angle() const { return rotation_angle_; }
   float force() const { return force_; }
+  int source_device_id() const { return source_device_id_; }
 
   // Relocate the touch-point to a new |origin|.
   // This is useful when touch event is in X Root Window coordinates,
@@ -471,6 +473,9 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
   // Used for unit tests.
   void set_radius_x(const float r) { radius_x_ = r; }
   void set_radius_y(const float r) { radius_y_ = r; }
+  void set_source_device_id(int source_device_id) {
+    source_device_id_ = source_device_id;
+  }
 
   // Overridden from LocatedEvent.
   virtual void UpdateForRootTransform(
@@ -504,6 +509,9 @@ class EVENTS_EXPORT TouchEvent : public LocatedEvent {
 
   // Force (pressure) of the touch. Normalized to be [0, 1]. Default to be 0.0.
   float force_;
+
+  // The device id of the screen the event came from. Default to be -1.
+  int source_device_id_;
 };
 
 class EVENTS_EXPORT KeyEvent : public Event {